1. Field of the Invention
The present invention relates to an analog accumulator, more especially an analog accumulator used in association with a solid state image analyzer for averaging and storing the fixed pattern noise FPN.
In fact, in solid state image analyzers there frequently exists non uniformity of the dark signal which is reproduced in each line. This non uniformity or fixed noise is generally called FPN. To eliminate this non uniformity which causes a defect on the images to which the eye is very sensitive, it is stored and subtracted from each line of the video signal.
2. Description of the Prior Art
This operation is at present carried out using digital circuits in the following way:
the video signal of one or more black lines placed at the beginning of the frame, namely the parasite signal FPN, is digitalized using an analog-digital converter then stored in a random access memory. This memory is then read for each line, converted into analog form using a digital-analog converter and the parasite FPN signal is subtracted from the video signal so as to give a corrected video signal free of FPN.
The disadvantage of this technique is that it introduces a noise relative to time called "rain noise" which results from storing the time related noise during acquisition of the fixed noise called FPN.
Consequently, different improvements have been made to the basic circuits for reducing the amplitude of the "rain noise". One or these improvements is described, for example, in the European patent application published under n.degree.89 203 in the name of Sony Corporation and consists in adding a level selector. However, these improvements increase the complexity so the manufacturing cost of the basic circuit which already comprises an analog-digital converter, a random access memory and a digital-analog converter.
Furthermore, when the parasite signal to be subtracted, mainly the FPN, is constant or slowly varying during the frames, the rain noise may be reduced by accumulating and averaging several fixed noise of FPN lines. In fact, accumulation of several FPN lines improves the FPN/rain noise ratio by a factor .sqroot.M, if the accumulation takes place over M lines.